Rays' Hope: Good Science Defends Maligned Predator

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Sweet Briar College researchers investigate cownose rays' dietary habits to protect it from human competitors.

Doreen McVeigh (center), a researcher with Hood College and recent Sweet Briar College graduate, and Sweet Briar undergraduates McKenzie Grundy and Maryanne Grey release a newborn stingray pup into the Chesapeake Bay.

I have seen these beautiful rays shot with guns and shot with arrows and had large stones and cinder blocks dropped on their heads, all because everyone assumes that they are a direct competitor that needs to die.

Maryanne Grey ’12 chooses lunch carefully on days that she is working on her senior independent study research. Sorting through the semi-digested contents of cownose ray stomachs and intestines is smelly work.

“I try to eat something that if I was to get sick, I wouldn’t mind it coming back up,” she says.

Once past the odor, it’s easy to get excited by what she finds. Much of it is too small to easily identify but the discovery of three quarter-sized crabs is pay dirt. Going into the project, Grey expected the rays’ diet to consist of bigger, high-caloric prey such as clams and crabs. After examining 20 or so stomachs, that’s not what she is finding.

Cownose rays are those sleek kite-shaped, long-tailed fishes you often see swimming around in aquaria “touch tanks.” Seems odd, then, that life in the Chesapeake Bay is considerably more hostile for the stingrays.

“I have seen these beautiful rays shot with guns and shot with arrows and had large stones and cinder blocks dropped on their heads, all because everyone assumes that they are a direct competitor that needs to die,” says Grey’s advisor John Morrissey, a marine biologist at Sweet Briar College.

The rays have been accused of a host of atrocities from wiping out the softshell clam fishery and plundering commercial oyster beds to destroying delicate grasses that conservationists have been at pains to re-establish in the environmentally troubled Chesapeake.

The problem is they migrate into the Bay every summer by the millions. There they both reproduce and feast — leaving behind telltale feeding craters in the sediment where they’ve searched for prey. In such numbers, and with each adult weighing 25 to 35 pounds, they do eat a lot. But what exactly are they eating?

“Well, everyone in the area simply assumes that they are eating whatever is important to that person,” Morrissey says. “So commercial clammers are ‘certain’ that the rays are eating clams, and recreational crabbers are ‘certain’ that they are eating crabs, and the oystermen are ‘certain’ that they are eating oysters. Bottom line? Everyone hates them and a grassroots campaign to exterminate them is vigorously under way.”

Worse, there’s a movement afoot to establish a commercial fishery to control their numbers in the Bay – something scientists fear would endanger the migratory species and have ecological repercussions. The females give birth to only one pup a year, there’s no reliable estimate of their actual numbers and no one’s sure where they go when they leave the Chesapeake in September.

To take some of the heat off the beleaguered fish, Grey and Morrissey are collaborating with researchers at Hood College to determine what it really does eat. Previous studies produced conflicting data, but one in the late 1970s concluded they eat softshell clams exclusively. It was later supported by a 1985 study. That fishery, however, no longer exists in the Bay.

In the summer of 2010 Grey and her colleagues spent several weeks in St. George Island trying to catch live rays to examine their stomach contents. Eventually they began collecting animals that are discarded by fishermen when they wander into commercial pound nets.

Under Morrissey’s guidance in the lab, Grey is working with frozen specimens this semester, and will report on her findings in late November. They may not be ready to draw conclusions by then; much depends on how many samples Grey is able to process.

So far, however, the diversity of worms, fishes and nearly microscopic bivalves contradicts earlier studies suggesting their diet is highly specialized. That could go either way for the cownose ray.

“Frankly, I would be shocked if they don’t eat something that is of commercial value,” Morrissey says. “But if we can show that they are still very narrow in their prey selection, then maybe we can cause, for example, oystermen to continue to despise them while everyone else leaves them alone.

“Would the rays get to swim around with fewer arrows in their backs after that?”

For local TV coverage of Grey’s research, see the story on WSET ABC 13.

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Jennifer McManamay
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